Email alerts are sent twice

Steps to reproduce:

1. Update zabbix to 4.0 version
2. Trigger a problem with an associated action that sends email alerts

Result:
E-mail is sent twice for the problem and twice for the resolution. Other media types doesn't seem to be affected.

Expected:
E-mail should be sent once for the problem and one for the resolution.

This worked as expected before the update. Same behaviour in two different servers.